In the manufacture of rubber printing plates, a pattern of the characters or material to be printed is produced and a mold is then produced from the pattern. Subsequently, a rubber printing plate is cast from the mold.
Photosensitive resins have been used extensively in the production of patterns. In one system, an uncured photo-sensitive resin is bonded to the surface of a metal backing plate and a photographic negative, bearing the characters to be printed, is applied to the surface of the resin and exposed to ultra-violet light. The areas of the resin exposed through the photographic negative are crosslinked or cured to provide a hardened structure, while the unexposed areas remain soft. Subsequently, the uncured areas can be removed through use of a liquid wash-out solution to provide the pattern.
The photosensitive resin bonded to the metal backing plate is sold in relatively large sheets and is cut to size for the particular printing plate to be produced. The advantages of this type of photosensitive resin system are offset by the high cost of the composite sheets.
Other systems have been used in the past utilizing liquid photosensitive resins. With these systems, a layer of the liquid resin is layed onto a horizontal glass plate, while a photographic negative is applied to the undersurface of the plate. An ultra-violet light unit, located beneath the plate, is employed to crosslink the areas of the resin exposed through the photographic negative. After exposure, the uncured areas of the liquid resin can be readily washed away with a wash solution to provide the pattern.
This liquid resin system has certain inherent disadvantages in that it is limited to a relatively thin resin layer so that the pattern has limited usage and cannot be used to produce printing plates requiring substantial depth of characters, as is necessary when printing corrugated paperboard.
In producing patterns from photosensitive resins, certain portions of the resin will require a longer time of exposure to ultra-violet light than other portions. For example, small characters, such as commas, periods, and the like, require a greater time of exposure than larger characters. Because of this, the normal procedure is to expose the entire negative for a given period of time and then blackout those areas which have had the desired amount of exposure and continue to expose those areas requiring additional time. With a liquid resin system as applied to a horizontal glass plate, it is difficult, is not impossible, to black out certain areas of the negative to vary the time of exposure due to the inaccessible location of the negative.